The present invention relates generally to medical devices and particularly to delivery systems for self-expanding medical devices.
Minimally invasive medical devices have become relatively common devices for treating numerous organs, such as the vascular system, colon, biliary tract, urinary tract, esophagus, trachea and the like. Stents are one type of minimally invasive medical device that is particularly useful for treating various conditions including occlusions and other related problems that restrict flow through a passage (generally referred to as a stenosis). Stents are also useful for treating various types of aneurysms. Although stents are a well-known type of medical device, many other types of medical devices are used for treating conditions within the human body.
Self-expanding medical devices offer several advantages for treating conditions within a body. In general, self-expanding medical devices are made from spring-like materials, such as stainless steel and nitinol. Typically, self-expanding medical devices have a support structure made from a spring-like material that is compressible into a collapsed configuration. The medical device is usually restrained in the collapsed configuration by a restraint, such as an outer catheter. In a minimally invasive medical device, the collapsed configuration is designed to allow the medical device to be passed through the body in a small profile delivery system that minimizes trauma to the body during the procedure. Once the medical device is positioned at the site in the body where the medical device is intended to be released, the restraint is removed from the medical device, and the spring-like properties of the support structure cause the medical device to expand to a larger configuration. If a medical device is intended to be left in the body for an extended period of time, the delivery system is then withdrawn from the body, and the medical device is left at the site where it was expanded.
One challenge with delivering self-expanding medical devices is that the medical device exerts outward force against the restraint when the medical device is in the collapsed configuration due to the spring-like properties of the support structure. This can make it difficult to remove the restraint at the implantation site. For example, in one type of delivery system where the restraint is an outer catheter that is withdrawn from the medical device, the medical device typically exerts outward force against the inner surface of the outer catheter. This creates friction between the medical device and the outer catheter that must be overcome in order to release the medical device. Friction between the medical device and the restraint can be particularly high for medical devices that are especially long; for medical devices with coatings or graft layers with higher friction coefficients; and for restraints that do not have low friction coefficients.
Because of the tension and/or friction that occurs between a self-expanding medical device and its restraint, self-expanding medical device delivery systems suffer from a number of problems. In some cases, the amount of force that may be needed to overcome the friction between the medical device and its restraint can be so high that a physician cannot release the medical device at the implantation site. The high forces needed to deliver self-expanding medical devices can also contribute to inaccurate placement of the medical device due to inadvertent movements of the delivery system that occur when the physician is attempting to overcome the delivery forces. Damage can also occur to parts of the medical device, such as the support structure, coatings and/or graft layers. It is also possible that the restraint can be damaged when trying to remove the restraint from the medical device.
Accordingly, the inventor believes it would be desirable to provide a self-expanding medical device delivery system with lower delivery forces.